fn foo(x: &mut u32, y: usize, f: fn(usize)) -> u32 {
    *x = 42;
    f(y);
    *x // can we optimize this to 42?
}

fn main() {
    let mut x = Box::new(0);
    let x_ptr = &mut *x as *mut u32;
    foo(&mut *x, x_ptr as usize, |y| unsafe { *(y as *mut u32) = 23; } );
}